Inkjet printing technology is used in many commercial products such as computer printers, graphics plotters, copiers, and facsimile machines. One type of inkjet printing, known as “drop on demand,” employs one or more inkjet pens that eject drops of ink onto a print medium such as a sheet of paper. The pen or pens are typically mounted to a movable carriage that traverses back-and-forth across the print medium. As the pens are moved repeatedly across the print medium, they are activated under command of a controller to eject drops of ink at appropriate times. With proper selection and timing of the drops, the desired pattern is obtained on the print medium.
An inkjet pen generally includes at least one drop-generating device known as a printhead, which has a plurality of nozzles or orifices through which the drops of ink are ejected. Adjacent to each nozzle is a firing chamber that contains the ink to be ejected through the nozzle. Ejection of an ink drop through a nozzle may be accomplished using any suitable ejection mechanism, such as thermal bubble or piezoelectric pressure wave to name a few. Ink is delivered to the firing chambers from an ink supply. The ink supply can be wholly contained within the pen body. Such an ink supply is considered to be “on-board” as the whole ink supply is carried on the carriage. With this arrangement, the entire pen, including the printhead, is replaced when the ink runs out.
In “off-board” or “off-axis” printing systems, the ink supply can comprise a stationary ink container located separately from the pen. The ink container is fluidly coupled to a chamber in the pen body via a fluid delivery system, which typically includes flexible tubing. Printing fluids other than ink, such as preconditioners and fixers, can also be provided. Off-axis printing systems often include multiple ink or fluid containers and multiple pens and printheads. The stationary position and relatively easy access of an off-axis supply can allow for relatively large volumes of printing fluids to be stored and delivered. The use of replaceable fluid containers that are separate from the printhead allows the containers to be replaced without replacing the printhead. The printhead is then replaced at or near the end of printhead life, and not whenever a container is replaced. An off-axis supply also provides for a lighter pen and carriage assembly. This generally requires relatively less energy to move, while moving faster, quieter, and/or with less vibration.
A concern with printing systems is that during shipping the system can be exposed to freezing temperatures, which could cause printing fluid in the system to freeze. Because most printing fluids contain water, they expand when freezing. This expansion can damage the fluid delivery system, such as causing the tubing to burst. One approach to avoiding such damage is to ship the printing systems without printing fluid. However, this approach creates certain logistical problems. For one, it is usually desirable to test a printing system at the factory prior to shipping to a customer. Such testing requires that the printing system be fully wetted. However, it is difficult and not cost efficient to wet a new printing system, test it, and then remove all of the printing fluid prior to transportation. Also, the occasion may arise where the user needs to return the printing system, such as for service or at the end of a lease. In this case, it is impractical to drain the printing fluid from the system prior to reshipment.